The present invention relates to a digital mobile communications system wherein mobile stations and a control station communicate via radio base stations each being located in a particular service area on a TDMA (Time Division Multiple Access) basis. More particularly, the present invention is concerned with a frame synchronization method among the radio base stations.
A TDMA digital mobile communications system of the type described usually has a control station and a plurality of radio base stations each being located in a particular service area. A number of mobile stations mounted on motor vehicles, for example, each communicates with the control station via one of the radio base stations existing in the service area where the mobile station itself is located, by using an assigned carrier and an assigned time slot.
When a given mobile station moves from a first service area covered by a first radio base station to a second service area covered by a second radio base station, it will communicate with the control station via the second radio base station thereafter.
Assume that when the mobile station moves as stated above, TDMA frame synchronization has not been established between the first and second radio base stations and the communication of the mobile station has to be handed over to the second radio base station. Then, the mobile station has to set up frame synchronization with the second radio base station before restarting the communication. To set up frame synchronization rapidly, TDMA frame generators installed in the individual radio base stations for generating TDMA frames may be reset at the same time.
Resetting a plurality of TDMA frame generators simultaneously as mentioned above may be implemented with a satellite which sends a reset signal to the individual radio base stations. Specifically, on receiving the reset signal, each radio base station resets the TDMA frame generator thereof for setting up frame synchronization. Such a system, however, is large scale and not practical since it needs not only a satellite but also a reset signal transmitter mounted on the satellite and a receiver built in each radio base station for receiving the reset signal from the satellite.
Thus, it has been difficult to set up frame synchronization among radio base stations without scaling up the system. Every time handover occurs, a mobile station has to set up frame synchronization with the TDMA frames being transmitted from a radio base station located in the new service area. Voice is interrupted until such frame synchronization has been established.